Estimating Your Due Month

Hi Mumma Berries!

For those of you who use FAM, NFP, or another mode of fertility tracking, did you choose to estimate your due month from your confirmed ovulation date, or did you go by LMP for the sake of being in sync with your healthcare provider??

We are TTC #1 at the moment, and this has been in the back of my mind. I automatically calculate off ovulation because I think the LMP theory is outdated and unclear given how variable each woman’s cycle can be, however I also realise that most healthcare providers don’t think this way. So I’m curious as to what others have done?

Also, bonus question for those that have had babies, which calculation did you find was more accurate??

:blush: Xx

I used my suspected date of ovulation based on my chart to calculate my duedate. However, due to the length of my cycle, the day was only “off”/later by 2 days, I think. If you have irregular cycles or long cycles it can make such a huge difference. I gave birth at 40 and 5, which is the exact average for first time moms. I loved Evidence Based Birth’s article on due dates.

There is a difference because I chose to have a homebirth and my provider was already very much knowledgeable about due dates, the differences in methodology, etc. An expected due month is so much more realistic. I also knew I was not going to be induced for my date, even if we went significantly “over” (I just would have been doing non-stress tests), so I didn’t have to fret, but I know many moms for whom a difference in calculation has made a big difference and they have had to fight with their providers to not be induced early when they think their babies are much littler than the LMP would suggest or that they are barely term when they are reaching 41 or 42 weeks when their provider is getting nervous and want to induce.

I could be totally wrong since this was not the case in my pregnancy, but if you choose to have an early ultrasound, my understanding is that they will almost always date the pregnancy based on that ultrasound and not based on LMP. However, anything later than 10 weeks will not be accurate to the day and it is an awful idea to change duedate based on ultrasound past that point (there is another EBB article about this).

I’d also say if you wind up with a provider who does things in a completely non-evidence based way and you actually talk about it and they won’t budge or think it is a silly thing to nitpick… they probably aren’t the right provider and probably have other non-evidence based relics of bygone medical training that will come out, too. It can be a good indicator of somebody’s philosophy of care if you bring up, “I’d rather consider this date as my duedate because I used FAM and know with accuracy which date I likely ovulated,” and they look at you like you’re crazy or they scoff or tell you no. Always pay attention to the way your provider makes you feel and go elsewhere if you are getting anything other than compassion from them.

Overall, I would say it is a better litmus test for your chosen provider than it is something that is likely to make a huge difference in your pregnancy unless your cycles are widely variant.


Back in the dim dark ages when I had my 5 children, the only way was to guess due dates, was to count 40 weeks from date of last period. (I live in Australia), The doctors and all the ladies used to tell young people like me, that babies can be two weeks before/after due date and still be full term. Very few babies are born on the due date anyway.

I once read in a book when the heroine was asking a native woman when baby was due, the mother said “[name_u]Baby[/name_u] come when ready”… I thought this was a great answer.

When scans became available for my two sons, the scan predicted due date only varied by a day or two.


I gave my midwife an estimated day of conception :weary: I.e- it could have been conceived on this day according to my cycles app where I kept a record when we were TTC. probs TMI but she was happy enough with that.

When I had my scan done at ~12 weeks it gave me an EDD that worked out 3 days before the EDD I’d figured out with my midwife based on timings :blush: However the scan also gave a stated a variation of ± 7 days (probably because baby has been measuring small all along) so my midwife decided to stick to the date we’d figured out rather than changing it to the scan date since our EDD fell within the range suggested by the scan.

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I’ve always found this so interesting. I tracked my ovulation with LH test strips and used a app to record it with my first. According to my midwife (using LMP date) my son was born at 42 weeks. According to my records he was born 40 weeks and 1 day from date of conception. Im pregnant with my 2nd now, and tracked everything the same way. [name_f]My[/name_f] midwife is using LMP again. We’ll see the outcome this time! I typically just go with my providers way of tracking, unless at 42 weeks they had wanted me to induce or some intervention. Thankfully I was with midwife and after a quick scan to check on amniotic fluid levels and all was OK, she let me go to 42 weeks.

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I’ve always loved the saying “babies are born on their birthdays, not due dates”.


100%!! I love this too :grin:

I was so into due dates before I went two weeks over with my first and spent 15 days cursing every second I did not have a baby in my arms. Then I gave up on them. Haha!

For what it’s worth, my ovulation and LMP due date calculations were the same. The first doctor I saw somehow calculated my first’s due date a week earlier. I’m still not entirely sure where she got that date. [name_f]My[/name_f] 12 week scan calculated I think 5 days earlier (baby ended up being big). [name_f]My[/name_f] midwives were actually excited that my LMP date was a bit later and went with that, so that if I went overdue I would have extra time to go into labor on my own. Didn’t happen but at least I had a chance. Due [name_u]January[/name_u] 2, born [name_u]January[/name_u] 17. Much to my dismay.

With my second every single thing lined up for a [name_u]March[/name_u] 26 due date (even the scan… even though he ended up being a giant…) I told everyone he was due in [name_f]April[/name_f] and convinced myself that was the case. I would advise everyone add two weeks to their due dates in their heads, so if they come earlier it’s a bonus and if they don’t you’re not severely disappointed. We ended up inducing the day after my due date due to his size and my history of being 2 weeks overdue, so not sure if I would have gone on my own or not.

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So interesting to read everyone’s experience. I’ve been reading up on gestational age vs conceptional age, and it’s so interesting. I didn’t previously know that the word “gestation” implied counting based off LMP.

I went of my ovulation days I also think when you are in the scan room and they give you the date that’s a big clue as they obviously take the baby’s measurements etc.

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And another thing I do think that week 40 is just a benchmark really. For me in my pregnancy I start to relax after week 38…for the baby to be born. [name_f]My[/name_f] first was born 39 +4 he looked very much ready to be out. He had really long fingernails and he was 8lb 8 which was quite big for my frame. ( He didn’t obviously reach the 40 week gestation) but he looked more than ready to be out if that makes sense?
[name_f]My[/name_f] second I was induced. It was a positive induction but I was induced because he had reduced movements and my pulse was too high.
I was induced at 38 weeks and he was born 38+2. He weighed 7lb 14 and he had vernix in some areas on his body which told me he probably could have done with an extra week in the womb. He also had a cute little jaundice nose.
With a third now I definitely feel it’s important to keep them in their for as long as possible ( I know it can’t be helped if baby did come early)
But for me beyond week 39 just feels right. [name_m]Even[/name_m] if you are totally exhausted and impatient in third trimester lol

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Two of my babies came on their own (one was induced a few days early) and they came right on their ultrasound due date (which was ahead of the one the doctor used based on my LMP). With my first two, they adjusted the due date to what the ultrasound gave, but my last baby the doctor refused and went off LMP right to the end (saying just because she was big didn’t mean anything). Well, she came right on the day the ultrasound gave just like her brother. So I don’t know if the scans are typically accurate or not, but next time I will be paying extra attention to what date they give! In case baby comes on that day again!

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My doctor initially estimated due date based on my LMP then adjusted it once I had an ultrasound. I know I typically ovulate a little late and this was confirmed when the ultrasound pushed back my due date by 4 days :slight_smile: Of course, that’s not really a huge deal, but if you have long or irregular cycles then it can really make a difference!

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